This invention relates to an aircraft structure. In particular, this invention relates to a heated leading edge component for an aircraft.
Leading edge components of an aircraft such as a wing slat or propeller are generally heated to reduce the build up of ice which could interfere with the operation of the component to the determent of the safety of the aircraft.
In some known designs, hot gas is bled from the engines of the aircraft and redirected to blow over the leading edge component for heating purposes. In an alternative heating system, heater mats can be attached to a surface of the leading edge component for heating the component. Heater mats of this kind are known in the art and generally comprise a laminar electrically resistive element through which a current can be passed for producing Joule heating for heating the leading edge component.
Leading edge components such as a wing slat typically include an outer skin which is aerodynamically shaped for the function of the component. An example is shown in FIGS. 1 and 2. FIG. 1 shows a wing 12 of an aircraft 10 with a wing slat 14. FIG. 2 shows a cut away view of the wing 12 shown in FIG. 1. In this example, the wing 12 includes a box portion 16 which is substantially rigid and which provides structural strength for the wing 12. The box portion 16 can also house one or more fuel tanks. The rear of the wing 18 generally includes wing components such as flaps. In this example, toward the front of the wing 12, a leading edge component, namely a wing slat 14 is provided. The wing slat 14 includes an outer skin 24 and is supported by a number of ribs 30. A plurality of such ribs can be provided along a length of the wing slat 14 for providing structural strength. The wing 12 can also include further elements such as a Kruger flap 20, which in this example can pivot (as shown generally by the arrow labelled A in FIG. 2) out from the wing for the modifying the aerodynamic characteristics of the wing 12.
As is shown in FIG. 3, the supporting ribs 30 of the wing slat 14 can be attached at right angles to the outer skin 24 for providing structural support. In FIG. 3, the attachment of the rib 30 can be achieved by providing the rib with a flange 32, which can itself be attached to the outer skin 24 using means such as an adhesive or sealant 34 located in a layer substantially in-between the flange 32 and the outer skin 24, and/or using a plurality of rivets 18 which pass through the flange and the outer skin 24. In some alternative designs, a L-shaped rib can be employed in an arrangement which effective comprises a flanged rib with a flanged portion only extending to one side of the rib, unlike the T-shaped ribs shown in FIGS. 3 to 5.
As is shown in FIG. 4, an arrangement of the kind shown in FIG. 3 can be provided with means for heating the leading edge component. These means can include one or more heater mats 40, which can be mounted on the outer skin and/or mounted on the flange 32.
FIG. 5 shows an alternative heating arrangement, in which a heater mat 40 is mounted directly on the outer skin 24 and the flange 32 is then mounted on top of the heater mat 40.
There are a number of problems associated with the mounting arrangements described above in relation to FIGS. 3 to 5.
Firstly, it should be noted that where attachment of a flange 32 to the outer skin 24 is provided for using rivets or similar means, the region of the flange 32 labelled “Z” in FIG. 3 is in fact mechanically redundant as it does not provide any additional support for the rib. Instead, the region Z of the flange 32 serves only to transfer the load between the rib 30 and the attachment means for example rivets 18. Similar considerations apply in the case of L-shaped ribs.
Secondly, and with reference to FIG. 4, where heaters such as heater mats 40 are provided on either side of the flange and also on top of the flange 32, this can complicate the control system used to regulate the temperature of the outer skin 24. This is because the heaters 40 which are provided on the flange 32 need to operate at a higher power output than the heaters 40 which are provided on either side of the flange 32, since the former heaters must heat the outer skin 24 via the material of the flange 32, while the latter heaters can apply heat directly to the outer skin 24. This substantially complicates the heating arrangement and the control system which is used to regulate the power to the heaters.
Moreover, the fact that the heaters 40 mounted on the flange 32 require additional power increases the energy requirements for the heater system. Additionally, the higher temperatures at which the heaters 40 on the flange 32 need to operate may be damaging to the materials of the rib 30, the flange 32 and/or the adhesive layer 34.
Regarding the arrangement shown in FIG. 5, while the heater 40 is continuously applied to the outer skin 24 whereby good heating can be achieved, there are may structural and safety issues associated with this design.
For example, since the heater 40 is located adjacent the adhesive layer 34, heat degradation of the adhesive may occur—this is especially dangerous in the event of overheating in the heater. Over heating may be due to, for example, a failure in the power control system. In another example, a failure in the heater can occur if the leading edge suffers some kind of mechanical impact such as hail stones, bird strike or service vehicle accidents. Such damage can cause rupture of the heater and lead to local overheating until such time the control system can detect the failure and shut down that part of the system.
Furthermore, the interposition of the heater mat 40 between the flange 32 and the outer skin 24 inhibits effective and reliable attachment of the flange 32 to the outer skin 24. Also, maintenance of the heater 40 is made complicated by the fact that the portion beneath the flange 32 cannot be accessed without removing the rib 30.
This invention has been made in consideration of at least some of the problems indicated above.